Fig. 1 Layout of the NH3 synthesis device bearing the Nb-SrTiO3 photoelectrode loaded with Au-NPs and a Zr/ZrOx thin film

Ammonia has gained attention as a next-generation energy carrier. By combining an optical antenna structure that can concentrate light into a nano-space, and a co-catalyst that selectively adsorbs nitrogen, we have succeeded in selectively synthesizing ammonia from water and dinitrogen under visible light irradiation.
The synthesis of ammonia, which is a raw material for chemical fertilizers and chemical products, uses more than 1% of the energy consumption of the world for its synthesis. Therefore, this method of artificial photosynthesis that efficiently uses visible light can make a great contribution to energy savings on a global scale.

At this time this research group has developed a co-catalyst that can convert dinitrogen into ammonia with good efficiency, and by supporting this co-catalyst on a photoelectrode in which gold nanoparticles are loaded, they have succeeded in using water, dinitrogen, and visible light to selectively synthesize ammonia, which has gained attention as a next-generation energy carrier.

The research and development of artificial photosynthesis systems that convert solar energy into storable chemical energy have gained attention as a way to solve the global energy problem.

Ammonia is expected to be one of the next-generation energy carriers of chemical energy because it has little danger of combustion or explosion and can be liquefied relatively easily.

Fig. 2 The irradiation time-dependence of NH3 formation on the cathodic side of the chamber (blue diamond), O2 evolution on the anodic chamber (red square), and ratio of NH3 and O2 (green triangle), respectively.

At the present time ammonia is commercially manufactured by the method called the Haber-Bosch process1, but this reaction requires a large amount of energy, and more than 1% of the world’s energy consumption goes into the Haber-Bosch process.

Accordingly, in order to efficiently use ammonia as an energy carrier, the world awaits the development of a low-energy synthesis method that is fundamentally different from conventional synthesis methods.

Anticipated Outcomes

By combining an optical antenna structure that can concentrate light into a nano-space, and a co-catalyst that selectively adsorbs nitrogen, we have succeeded in synthesizing ammonia from nitrogen and water by using visible light, which is found in abundance in solar energy.

In the future, by improving the reaction efficiency and widening the response wavelength band, we will work towards the commercialization of this ultimate, clean “ammonia photosynthesis”, which can generate ammonia from the visible light in sunlight, the nitrogen in the air, and water.

Terms

Haber-Bosch process: A method of ammonia synthesis, for which the German chemist Fritz Haber had first been successful in research at laboratory scale, and which then was commercialized in 1913 by Carl Bosch of BASF. This process synthesizes ammonia (NH3) by using iron as a catalyst to react nitrogen (N2) and hydrogen (H2) at high temperature and high pressure. Since the extremely severe reaction conditions of 400ºC to 600ºC and 200 to 400 atmospheres are required, this process consumes an inordinate amount of energy. In addition, within the steps of the Haber-Bosch process, about 90% of the energy is consumed by the manufacture of hydrogen gas from fossil fuels.